1. Field of the Invention
This invention relates generally to a battery state estimator for cells in a battery pack and, more particularly, to a hybrid battery state estimator which combines an electrochemical solid-state concentration model with an empirical equivalent-circuit model, where the hybrid model is computed using an extended Kalman filter routine to provide values of open circuit voltage and other battery parameters which are accurate even under high-current operating conditions.
2. Discussion of the Related Art
Electric vehicles and gasoline-electric or diesel-electric hybrid vehicles are rapidly gaining popularity in today's automotive marketplace. Electric and hybrid-electric vehicles offer several desirable features, such as reducing or eliminating emissions and petroleum-based fuel consumption at the consumer level, and potentially lower operating costs. A key subsystem of electric and hybrid-electric vehicles is the battery pack, which plays a large part in dictating the vehicle's performance. Battery packs in these vehicles typically consist of numerous interconnected cells, which are able to deliver a lot of power on demand. Maximizing battery pack performance and life are key considerations in the design and operation of electric and hybrid electric vehicles.
A typical electric vehicle battery pack includes two or more battery pack sections, with each section containing many individual battery cells as needed to provide the required voltage and capacity. In order to optimize the performance and durability of the battery pack, it is important to monitor the state of charge and other parameters of the battery cells. State of charge of a cell is typically determined based on the open circuit voltage of the cell, using a relationship which is defined in the form of a state of charge vs. open circuit voltage (SOC-OCV) curve. However, open circuit voltage can only be directly measured when a battery cell has equilibrated for several hours with no charging or discharging current, at which time open circuit voltage is equal to terminal voltage. During charging or discharging of an electric vehicle battery pack, and for some time thereafter, open circuit voltage can only be estimated based on measured terminal voltage.
One existing technique for estimating open circuit voltage from terminal voltage of a battery cell during charging or discharging is to use an equivalent-circuit model of the battery cell. An equivalent-circuit model uses empirically-derived circuit elements, such as capacitors and resistors, to model the battery cell's behavior. However, under high-current charging or discharging conditions, equivalent-circuit models typically lose accuracy, as diffusion effects cannot be accurately modeled under these conditions. A battery state estimator is needed which can accurately model battery cell behavior under real-world operating conditions, and which is computationally efficient enough to run in real time.